Posted
by
timothy
on Saturday August 01, 2009 @01:30PM
from the cheap-tricks-are-the-best-kind dept.

Soychemist writes "Print some thermochromic ink onto a sheet of paper, put metal heating elements on the other side, and you have a rudimentary color-changing display. Chemists in the Whitesides Group at Harvard think that the devices could be used to provide a simple readout from cheap medical tests and kits that check water for pollutants. In the past year, the same scientists have made a three-cent medical test and improvised a centrifuge with an egg beater. Their aim is to invent useful gizmos with everyday materials."

My first thought was "Oh neat, color electronic paper." Then I thought, "But if it needs to get hot (and stay hot for that matter), it would not be very useful for a portable device like an e-book reader."

Perhaps with some chemical tweaking they could develop dyes that work at much lower temps?

I was thinking about making a thermochromic display for a custom watch, actually. Didn't know if anyone else had tried it before, but I guess I'm on the right track. It's going to take a lot of batteries to power, but I only really want it to run for Burning Man and Maker Faire.

I'm going to see if I can screen print resistive ink onto a PCB to make the heating elements. Failing that, I'll just go with thin film SMT resistors. Anyone know if that has been done before?

I don't know if it's been done before, but SMT resistors will work quite well. Just make sure they're rated at the temperature the thermochromic ink needs to get to... if not, you could just solder some nichrome wire across the pads. You might want to consider heat sinking things so that it will be able to cool off quickly as well... a bit of a balancing act, that.

Most of the films are rated for temperatures close to room temperature, though you can get other ranges. I've used through-hole resistors as heating elements before and they do indeed work quite well. A 1-watt metal film resistor will generate enough heat at 3 watts to melt its own solder joints. Long before that happens, it gets hot enough to melt through nylon or spectra, which is what I was using it for - a thermal knife for a load release.

(2) Draw heating elements with a #2 (or, if you can find it, #1) pencil. You can get the resistance of a heavy graphite line pretty low; if that's not low enough, run traces along the long edges of each segment, instead of connecting to the ends.

Bah, you kids and your fancy electric printers. This thing is useful, boy! Why, back before we had "thermochromatic" ink, we used our feces. And we hated it.So be glad of this advance, sonny. And make sure you shake hands with your right hand.

That's just electric. From wikipedia: Electronics is a branch of science and technology that deals with the flow of electrons through nonmetallic conductors, mainly semiconductors such as silicon. It is distinct from electrical science and technology, which deal with the flow of electrons and other charge carriers through metal conductors such as copper. This distinction started around 1906 with the invention by Lee De Forest of the triode.
So, there you have it.

What temperatures are needed to change the ink? Obviously if it needs to be really hot that is bad since it might burn the person holding it, while cooler temperatures can be affected by the atmosphere around the page causing problems.
Would it make more sense to use a different material then paper to print the ink on to avoid accidental colour changes?

Now how about finding something much more useful to do with those clunkers besides just crushing them into scrap? The current plan -- Democratically inspired, of course -- is one of the biggest wastes of already refined and manufactured resources imaginable, and an outright giveaway to the autoworker union.

*theoretically* before a car gets crushed for scrap it's already stripped of everything useful. glass can be recycled, they'd be mad to not take out the catalytic converter (the platinum in a catalytic converter can fetch more money than the rest of the steel in the car is worth)...

crushing for scrap is so that the parts can be melted down and recycled, which is a lot faster than digging the iron up out of the ground and smelting it into steel... unless you were thinking that the cylinder head from a 1962 F